Drop down list has a SelectedValue which is invalid because it does not exist in the list of items.

I am getting this error yet I know it to be untrue.
This is the code:
ddlPartnerOrganisation.DataSource = agency.AgencyGetListOfEYDN();
  string temp = ddlPartnerOrganisation.SelectedValue.ToString();
 
 
ddlPartnerOrganisation.DataValueField = "AgencyID"
;
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